Terahertz waves are electromagnetic waves generally having a frequency of 0.3 THz to 10 THz, and are expected for basic fields such as physical properties, electrospectroscopy, bioscience, chemistry and pharmaceutical science and for applied fields of atmospheric environmental assessment, security, material test, food test and communication.
As a device for oscillating terahertz waves, it has been developed a backward wave oscillator (BOW) and photomixing in several hundreds GHz band and free electron energy laser, p-Ge laser and quantum cascade laser (QCL) for 1 THz or more. These devices, however, have problems in the miniaturization and improvement of output power.
On the other hand, it has been recently developed a device for oscillating a wide-band terahertz wave with an optical switch or rectification using a femtosecond laser as a light source and applied for time domain spectroscopy (TDS) or the like.
In addition to this, for generating terahertz wave using a non-linear optical crystal such as LiNbO3, it has been known methods of utilizing quasi phase matching (QPM) and of utilizing phonon polariton. Such methods are expected for applications requiring sources generating the terahertz wave having high temporal and spatial coherency.
It is further possible to generate wide-band terahertz wave by optical current using the femtosecond optical pulses as described above, so as to provide a generation system suitable for wide variety of applications.
Patent document 1 (Japanese Patent Publication No. H09-146131A) discloses a device of oscillating terahertz wave using a y-plate or z-plate of LiNbO3. That is, a light source irradiates a pump wave into a substrate and another light source irradiates an idler wave into the substrate. The pump wave (frequency ω1), idler wave (frequency ω2) and polariton (terahertz wave: frequency ωT) satisfy the law of conservation of energy (ω1=ω2+ωT) and law of conservation of momentum (noncollinear phase matching condition: k1=k2+kth), so that polariton stimulated scattering is observed. In this case, due to the scattering property, the frequencies of the idler wave and terahertz wave are decided depending on the angles α and θ with respect to the optical axis of the pump wave.
According to the method, typically, the phase matching condition is satisfied when an angle α of wave vector k1 of the pump wave 3 and wave vector k2 of the idler wave is 0.5° to 1° and the terahertz wave 7 was oscillated (wavelength of 100 to 300 μm, frequency of 3 THz to 1 THz) at a high efficiency. Further, it is described that the terahertz wave is oscillated at an angle of 65 to 66° with respect to the idler wave. In the case that a y-plate is used, the crystal orientation is different. Therefore, the pump wave and idler wave propagate on a plane perpendicular to the substrate at an angle α to generate terahertz wave at an angle θ with respect to the pump wave.
However, (1) the crystal has a refractive index as high as 5.2 with respect to sub-milli wave (terahertz wave) so that total internal reflection occurs between the crystal and air. It is thus impossible to draw the terahertz wave into the air both in the cases of the y-plate and z-plate. (2) Optical loss in the crystal is large. For example, the optical intensity of the terahertz wave is reduced to about 0.1 percent with respect to a propagation distance of 3 mm of the terahertz wave. For these problems, according to Japanese Patent Publication No. H09-146131A, a grating is provided on a side face of the substrate to enable the emitting of the terahertz wave into the air at a high efficiency.
According to Japanese Patent Publication No. 2002-72269A, an exciting laser light, having a single frequency is irradiated and an idler wave having a single frequency is used for optical injection to generate terahertz wave having a high output power and whose spectrum line width can be reduced. A silicon prism is, however, used for drawing the terahertz wave.
Further, according to patent document 3 (Japanese Patent application No. 2009-185768; Japanese Patent Publication No. 2011-059670A), the applicant disclosed that terahertz wave is made cut-off state in a substrate having a thickness of 20 μm or smaller and irradiated into air at a high efficiency.
According to optical rectification using femto-second pulses, terahertz wave is generated by differential frequency generation of two frequency components included in wide spectrum of the femto-second optical pulses themselves. At this time, various differential frequencies are generated at the same time among the spectrum of incident light. As a result, the spectrum of the terahertz wave is composed of wide band having a frequency width of about reciprocal number of the width of pulse time period of the femto-second optical pulses.
The terahertz wave generation utilizing femto-second optical pulses is fundamentally same as the parametric process as described above. As described in non-patent document 4 “Intense Terahertz Pulse Generation by Pulse Front Tilting” “Laser Research” 37(5), pages 345 to 349 (2009)), it becomes recently possible to generate planar terahertz wave at a high intensity by tilting the optical pulse front, which is expected in applications of imaging.
As described above, according to prior devices, when terahertz wave is generated in parametric oscillation, considerable portion of it is absorbed in the inside of a crystal. Moreover, as the refractive index of the crystal is considerably larger than that of air, it is not possible to draw the terahertz wave to the outside without providing a prism or grating on a surface of the crystal. The intensity of the terahertz wave, which can be oscillated from the device, is low and unpractical, and it is required a step of providing the prism or grating on the surface of the crystal. Further, accurately, it is further generated a propagating loss or reflection loss due to the prism or grating.
On the other hand, in visual light or infrared light, it has been tried to subject a surface of an optical part to fine processing to provide a three-dimensional structure which is provided regularly at an interval smaller than a wavelength of light, so as to provide the function of preventing reflection on the surface of the optical part (Patent documents 4, 5, 6 and 7; Non-Patent documents 1, 2 and 3).    (Patent document 1) Japanese patent Publication No. H09-146,131A    (Patent document 2) Japanese patent Publication No. 2002-072,269A    (Patent document 3) Japanese patent Application No. 2009-185,768A; Japanese Patent Publication No. 2011-059670A    (Patent document 4) Japanese patent Publication No. 2002-287,370A    (Patent document 5) Japanese patent Publication No. 2004-521,329A    (Patent document 6) Japanese patent Publication No. 2003-177,210A    (Patent document 7) Japanese patent Publication No. 2010-020,120A    (Non-patent document 1) “OPTICS LETTERS,” Vol. 24, No. 20, October 15, p 1422 “Optical Society of America”    (Non-patent document 2) ΠKONICA MINOLTA TECHNOLOGY REPORT┘ Vol. 2 (2005) pages 97 to 100 “Polymeric Wide-band Wave Plate Produced via Nanoimprint Sub wavelength Grating”    (Non-patent document 3) ΠSynthesiology┘ Vol. 1, No. 1 (2008) pages 24 to 30 “Challenge for Low-Cost Production of High-Performance Optical Devices—Realization of Sub Wavelength Grating Structure via Glass imprint method.”    (Non-patent document 4) “Intense Terahertz Pulse Generation by Pulse Front Tilting” “Laser Research” 37(5), pages 345 to 349 (2009))